Computer devices (desktop computers, laptops, smartphones, tablets) rely on a variety of sensors and system to determine they location on the globe: GPS, triangulation of signal strengths from nearby WiFi access points or cell towers, DNS, or IP Geolocation. The granularity of these methods can be as fine as a few meters in the best case (GPS when outdoors with clear signals from many GPS satellites) or as coarse as an entire town (IP Geolocation). In some uses cases, such gross geolocation estimates with coarse granularity may be adequate, such as for navigating roads or for signaling one's presence in a building. However, such gross geolocation estimates are too coarse to detect when devices are within social proximity of each other: for example, in the same room, at arm's length, or across-the-table from each other.
Near-Field Communication (NFC) ostensibly addresses the coarse granularity of today's positioning systems. NFC allows two devices to detect when they near each other using a technology similar to RFID tag readers. For the interactions described above, NFC has two shortcomings: (1) it requires devices to be almost in contact with each other to be detected, and (2) it cannot detect when multiple devices are near each other.